IN BRIEF

Astronomers have detected cosmic ray particles that hint at a number of recent, nearby supernovae. A growing body of evidence, including lunar samples and material from the Earth’s oceanic crust, seems to confirm the findings.

TINY IRON NUCLEI

New data from the Cosmic Ray Isotope Spectrometer (CRIS) onboard NASA’s Advanced Composition Explorer (ACE) spacecraft is hinting at an unexpectedly violent past for our solar neighborhood.

In fact, it suggests that one or more stellar explosions may have occurred nearby, and within only the last few million years—close and recent enough that our australopithecine ancestors may have beheld the cosmic spectacles in the ancient East African skies.

Cosmic “rays” are actually not rays at all, but are tiny charged particles—atomic nuclei stripped of their accompanying electrons and accelerated to relativistic speeds by enormous and incomprehensible energies that are, in many ways, only dimly understood.

One particular type of these nuclei, a rare iron isotope called 60Fe, is especially important, as it acts like a tiny clock, measuring by its very existence the age and thus the distance of the cosmic catastrophe that created it. 60Fe has a half-life of 2.6 million years, in other words, in any given sample of 60Fe nuclei, half of it will have decayed to a more stable element after 2.6 million years has elapsed.

It’s not a common sort of material; after 17 years in space, and after registering 300,000 galactic cosmic ray particles of ordinary iron nuclei, CRIS has only detected fifteen 60Fe particles.

But these small particles tell a big story.


A map of the solar neighborhood, showing the Sun in relation to the Scorpius-Centaurus Association, where the recent supernovae are thought to have originated. Credit: Linda Huff (American Scientist), Priscilla Frisch (U. Chicago)

THERE GOES THE NEIGHBORHOOD

The research, published in the April 14 issue of Science, is the most recent clue in a steadily increasing body of evidence that a supernova detonated near our Solar System in the past few million years, and polluted the system with the byproducts of the explosion.

According to Robert Binns, lead author of the paper, “[o]ur detection of radioactive cosmic-ray iron nuclei is a smoking gun indicating that there has been a supernova in the last few million years in our neighborhood of the galaxy.”

Morgan-Keenan spectral classification of stars.

Galactic cosmic rays are thought to come from clusters of massive stars called OB associations, named for the blue and blue-white light they emit. Our Sun in a G star in this classification scheme. (Image: Wikimedia Commons)

It’s thought that radioactive 60Fe nuclei are generated in the core-collapse explosions of giant O- or B-class stars, which tend to aggregate in immense star formation clouds and clusters called “OB Associations.” The gregarious nature of these stars is important to the formation of60Fe cosmic rays—the nuclei are first thrown into space from the explosion of one star, and then accelerated to relativistic speeds by the explosion of a second, nearby star shorty thereafter (within a few 100,000 to a million years).

It’s sort of a team effort.

What are cosmic rays?

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Cosmic rays were discovered before World War I but named in the 1920s by the famous physicist Robert Millikin, who called them “rays” because he thought they were a form of high-energy electromagnetic radiation.

But in the early 1930s, Arthur Compton, later chancellor of Washington University in St. Louis, organized a collaboration of eight research groups to measure cosmic-ray intensity at 69 locations around the Earth. Variations in the intensity with magnetic latitude showed that cosmic rays were deflected by the Earth’s magnetic field, and must therefore be charged particles (the nuclei of atoms stripped of their electrons) rather than electromagnetic radiation.

Of these nuclei, 90 percent are hydrogen nuclei (protons), 9 percent are helium nuclei and only one percent are the nuclei of heavier elements. But that one percent provides the best clues to how the particles are created.

Although energetic particles coming from our sun are sometimes called cosmic rays, astrophysicists prefer to call these comparatively low energy particles SEPs, or solar energetic particles.

They reserve the term “cosmic ray” for particles coming from outside our solar system, either from our galaxy or beyond. These fall into two groups based on their energies.

Most of those now thought to be created by supernova explosions in or near our galaxy have energies of 109 and 1010 eV — although they can have energies as high as 1015 or higher — and a flux of about one per second per square centimeter. (The molecules in our atmosphere have kinetic energies of about 0.03 eV.)

But there are also cosmic-ray nuclei that are several billion times more energetic, and much rarer. They have energies of 1019 to 1020 eV and fluxes more like one per square kilometer per century. The source of these extremely rare particles is still unknown.

Strangely enough, a number of recent studies have corroborated these findings—showing that there are heightened levels of 60Fe deposition in 2.2 million-year-old oceanic crust, and Apollo lunar samples also indicate that the Moon was spattered with 60Fe in recent history.

So it seems the explosions likely happened sometime between 2.2 and 2.6 million years ago. As for a culprit? Astronomers think the supernovae probably occurred in the Scorpius-Centaurus Association, roughly 400 light-years away, and most notable as the home of the red supergiant Antares—which may soon be the next supernova to bombard our Solar System with 60Fe nuclei.